The present invention relates to the reaction of hydrogen gas with transition metal alloys, and particularly to the rapid reaction at mild temperatures of hydrogen gas with alloys containing niobium or tantalum.
Most metals that form hybrides react very slowly in bulk form at room temperature with hydrogen gas. Metallic niobium, for example, is relatively inert in bulk form at room temperature in the presence of hydrogen gas, with the hydrogen only slowly dissolving in the body-centered cubic phase structure of the metal until saturated (the so-called alpha phase), and then additional hydrogen reacting only slowly to form a precipitated niobium hydride. Most other metals that form hydrides react in a similar fashion, with the rate of alpha phase formation and hydride formation varying among metals and alloys, but rarely occurring at room temperature in less than one hour. Attempts to increase this rate by plating over niobium with nickel or palladium or iron have been reported.
For many applications of metal hydrides, it is desirable to form the hydride from bulk metal, pulverize the hydride into some form of granular or powder structure, and thereafter cyclically remove hydrogen to form a lower hydride or the free metal and thereafter reintroduce hydrogen to form the original hydride. Starting with bulk metal or bulk alloy, it is normally necessary to go through an induction period, wherein the metal is heated to a temperature such as 300.degree.-700.degree. C., then reacted with hydrogen at high pressure and then cooling the system very slowly until a temperature below about 100.degree. C., and preferably about room temperature, is reached. At the higher temperature, the rate of hydrogen dissolving in the metal (the alpha phase) is increased so as to achieve saturation in a matter of minutes rather than hours or days. At the high temperature, however, the equilibrium hydrogen pressure is so high that relatively little hydrogen actually dissolves or forms hydride. Accordingly, it is only upon gradual cooling that saturated alpha phase forms, and thereafter hydrides form. While many metals require only a single induction process to form the hydride, with the subsequent hydride powder cycling at a reasonable reaction rate, it should be apparent that the induction process represents a distinct disadvantage in forming and utilizing metal hydrides.